Microminiature pointed, conical or pyramid-shaped tapered structures have been suggested for use, e.g., in scanning tunneling microscopes, in atomic force microscopes, as biological probes, and in vacuum-microelectronic devices. Also, more specifically with respect to the latter, wedge- or ridge-shaped structures have been suggested, e.g., as disclosed by R. B. Marcus et al., "A Novel Vacuum Microelectronics Electron Field Emitter", The First International Vacuum Microelectronics Conference, June 13-15, 1988, sponsored by The Electron Device Society of the IEEE.
With regard to specific electronic devices, micro-miniature tapered structures are receiving attention for use as cathodes in specialized applications; for example, as described by C. E. Holland et al., "Spindt Cold Cathode Vacuum Fluorescent Display", EuroDisplay 87, pp. 1-3, resulting structures are suggested for use as cathodes in matrix cathodoluminescent displays. Further use is contemplated, e.g., in transducers, in slow-wave structures for coherent terahertz electromagnetic sources, as cathodes for free-electron lasers, and in integrated electron gun structures for traveling wave tubes; see R. Greene et al., "Vacuum Integrated Circuits", IEDM 85, pp. 172-175. Proposed further are three-terminal field-emission devices for very-high-speed microelectronic circuits; see, e.g., U.S. Pat. No. 4,721,885, issued Jan. 26, 1988 to I. Brodie.
Since, unlike solid-state devices, field-emission devices rely on electrons traveling in free space, miniaturization of such devices poses challenges not encountered at present in the fabrication of solid-state devices. One fabrication method, disclosed by C. A. Spindt et al., "Physical Properties of Thin-film Field Emission Cathodes with Molybdenum Cones", Journal of Applied Physics, Vol. 47 (1976), pp. 5248-5263, involves the formation of cone-shaped emitter structures by molybdenum metal deposition in the presence of a suitable mask. Proposed also is a method involving selective etching of (doped) silicon; see U.S. Pat. No. 3,970,887, issued Jul. 20, 1976 to D. O. Smith et al. And, for yet other methods, involving molding of metal or semiconductor material, see U.S. Pat. No. 4,307,507, issued December 29, 1981 to H. F. Gray et al., and U. S. Patent 4,685,996, issued Aug. 11, 1987 to H. H. Busta et al.
For the fabrication of miniaturized tapered structures, and especially of arrays of such structures for use as field-emission cathodes, silicon technology is considered particularly advantageous; see, e.g., J. B. Warren, "Control of Silicon Field Emitter Shape with Isotropically Etched Oxide Masks", Second International Conference on Vacuum Microelectronics, July 1989, pp. 37-40 and D. Stephani et al., "Fabrication of Densely Packed Sharp Silicon Field Emitters Using Dry Etching Techniques", Second International Conference on Vacuum Microelectronics, July 1989, p. 18.
In the context of silicon processing--and, e.g., in the interest of maximizing field-emission efficiency in field-emission devices, or in the interest of increased resolution in scanning microscopes--it is desired to enhance the sharpness of micro-miniature tapered structures or protuberances. Also, where an array of such structures is included as, e.g., in a display device, high uniformity of sharpened protuberances is desirable. Uniformity is similarly desirable in the fabrication, by wafer processing, of device chips comprising protuberances It is such twin aims of sharpness and uniformity which motivate the invention described below.